1. Field of the Invention
The invention pertains to switching optical signals and more particularly to the compensation of temperature induced diffraction angle variations in switches.
2. Description of the Prior Art
High information transmission capacity, immunity to electromagnetic interference, and freedom from ground loop problems ideally suit optical transmission systems for linking distributed computers, computer controlled industrial components, and other data transmission systems. These optical transmission systems utilize optical fibers to serially link a multiplicity of optical repeater stations. A power failure at one of the serial link stations, however, may interrupt the data chain and cause the entire system to fail. To prevent such a catastrophe, a fail safe optical switch is employed at each repeater which operates to bypass that station when a fault occurs, as for example, a power loss. These fail safe switches must possess low insertion loss properties, and provide high isolation between the input and output optical fibers during the "Power On" mode. Many such networks have hundreds, if not thousands of data stations each requiring a bypass switch, making the cost of the by-pass a major factor.
Conventionally, the optical switches utilized have been mechanical in nature. Mechanical switches, though relatively inexpensive, inherently include moving parts and generally require high driving power. These moving parts are subject to wear, abrasion, fatigue and other mechanical stresses and as a consequence are themselves prone to failure.
Optical switches, utilizing a liquid crystal material, as the optical signal direction control mechanism have been proposed. At present, however, these proposed liquid crystal switches are expensive, temperature sensitive, and difficult to mass produce. As proposed, these devices employ a series of triangular prisms, having optically flat surfaces. These prisms are difficult to manufacture and represent the bulk of the manufacturing costs of the switch. Further manufacturing difficulty arises due to the requirement that the optically flat bases of the prism be parallel and laterally aligned to insure that the path of the light beams passing therethrough maintain a prescribed path.
Other types of optical switches in the prior art utilize a Faraday rotator comprising YIG crystal to effect polarization rotation of the optical signal and a polarization separator to accomplish the desired switching. These switches exhibit excessive inertia due to the wiring in an electromagnet required to establish the necessary magnetic field about the YIG to produce the polarization rotation. Additionally, large amounts of electrical current must pass through the coils to establish the required magnetic field. The current may be reduced somewhat with additional turns of wire, but this adds to the inertia of the switch. Further, the YIG crystal is construed as a slab optical waveguide and presents an interface problem with the optical fibers of the data system.
Another bypass switch of the prior art utilizes PLZT wafers to which an electrical voltage is applied to effectuate a polarization rotation. This switch, as do the other polarization sensor devices, requires polarization beamsplitters to direct the polarized light and collimating and focussing lenses for interfacing the PLZT wafers with the optical fibers. In addition to requiring the high voltage to provide the necessary polarization rotation, the PLZT wafers are difficult and expensive to manufacture. Further, the necessary electrode through which the wafer voltage is applied must be positioned on the wafer clear of the light path, adding to the cost and size of the manufactured switch.
An optical switch which overcomes many of the above-discussed deficiencies of the prior art is disclosed in U.S. patent application Ser. No. 07/245,593, now U.S. Pat. No. 4,902,087, of Stanley J. Lins, et al for "Fiber Optic Bypass Switch", filed Sept. 19, 1988 and assigned to the assignee of the present invention, herein incorporated by reference. In accordance with its principals, that invention includes an acoustic sensitive device, such as a Bragg cell, having an index of refraction that is variable in accordance with an applied acoustic signal. This acoustic signal may be provided to the device by an electro-acoustic transducer responsive to an electrical signal coupled thereto. Prior to the application of the electrical signal, light signals incident to an input port on one side of the device exit from an output port on the other side which is in-line with the input port. When an electrical signal is applied, the transducer provides bulk acoustic waves that fill the device and refract the light to a second output port, the position of which is determined by the refracted index change caused by the bulk acoustic waves. If two input ports are provided, the switch may be utilized as a bypass switch where, in the unenergized mode, light incident to the first input port is coupled to a first output port in alignment therewith, while in the energized mode, light incident to the first input port is coupled to a second output port displaced from the first output port. A second input port may be positioned adjacent to the first input port in such a manner that when the device is energized, light incident to the second input port is refracted to the first output port.
A bypass switch utilizing a Bragg cell will, in the energized mode, couple approximately 90% of the light from the first input port to the second input port, with approximately 9% being coupled to the first output port. Thus, only 10 dB of isolation is provided between the two output ports. In an embodiment of the invention disclosed in U.S. application Ser. No. 245,593, a reflector is positioned at what would be the first output port in such a manner that the light incident from the first input port is reflected therefrom to establish the first output port on the same side of the device as the first input port. In this manner, when the Bragg cell is energized, the approximate 90% of the light energy exits the second output port while the approximate 9% incident to the reflector is reflected therefrom to be refracted once again and to couple about 1% of the light originally incident to the first input port to the first output port, thereby providing approximately 20 dB of isolation between first and second output ports. Additional isolation may be provided between the first and second output ports by positioning a reflector at what would be the first output port with but a single reflection, thereby establishing a second reflection such that the light incident thereto from the first reflector is reflected to exit the Bragg cell at a first output port on a side opposite that of the first input port. Such a second reflection provides an isolation between the first and second output ports of approximately 30 dB. Further additional isolations may be provided by properly positioning additional mirrors along the sides of the Bragg cell.
Though the invention of U.S. application Ser. No. 245,593 operates satisfactorily within a limited temperature range, significant angular diffraction deviations are experienced when a temperature variation exceeding this limitation is encountered. These angular deviations with temperature may be eliminated by placing the switch in a temperature controlled oven to maintain the switch at a constant temperature. This solution, however, requires significant real estate, consumes excessive power, and adds appreciably to the initial and operating cost of a data transmission system.